Nowadays, transporting high-viscosity oil and the like through pipes, the viscosity of the moving oil has been reduced by electrical-heating of the pipe line in order to improve an efficiency of transportation of oil. According to the prior art, such electrical heating system is constructed, for example, by an electric heating cable consisting of a feeder line and an electric heater, respectively are covered by an inorganic insulating cover material, and a heat retaining material made of calcium silicate and the like, the electric heating cable is laid in straight or helically wound along a longitudinal direction of an oil pipeline, consequently heat loss through the pipe is prevented. According to other example of the prior art, a small-diameter pipe is welded on the oil pipeline and an electric heating cable is placed within the small-diameter pipe using a skin current heating effect in the pipe. It is called a SECT (Skin Effect Current Tracing) Method. Also, such electric heating cables are used to prevent roads, pedestrian sidewalks, and pedestrian crossing bridges in cold regions from freezing and being deposited of snow, or heat buildings. In case that the electric heating cables are installed in roads and sidewalks, the cables are buried in a serpentine configuration under the ground of 50-100 mm in depth and cement or asphalt concrete is cast in the direction of the serpentine cable.
Other than the electric heating cable above, heating tubes have been used in order to sustain heat of the fluid flowing through the tube and control the temperature of the fluid, which heating tubes are made by integrally constructing or assembling a tube, a feeder line, and the electric heater. The heating tube is usually made of a thermal resisting tube of metal or non-metal and a fluid flows therethrough, a feeder line extending along the thermal resisting tube, and an electric heater wound around the assembly of the tube and the feeder line helically or spirally in order to dielectrically cover the tube. Such a heating tube has been employed in, for example, a medical dialyser so as to sustain heat or temperature of the dialysis liquid flowing in the tube at a human temperature. The electric heater used in the electric heating cable and the heating tube generates electrically heat due to Joule effect to heat the target material.
Next, disadvantageous features of the conventional electric heater will be described with reference to FIG. 1 and FIG. 2(A), 2(B), respectively showing the construction of the electric heating cable and a heating tube formed according to the prior art.
As shown in FIG. 1 depicting an electric heating cable, it is constructed by a feeder line 1, two cores 2 of the feeder line 1, insulating covers 3, 4 and 9 made of insulation material, a resistance wire 5 wound around the insulating cover 4 spirally, and a shield 10. The resistance wire 5 is an electric heating means. Ordinarily, a diameter of the resistance wire 5 is AWG (American Wire Gauge) about 36-42. The two cores 2 are developed through an exposed portion 7 formed in the insulation cover 4. The cores 2 and the resistance wire 5 are connected each other at a connection point 8.
Such electric heating cables are attached to, for example, pipes of installations in cold regions. In operation of such electric heating cables, when an AC voltage is impressed to the cores 2, respective resistance wire 5 connected in a zone section between the two adjacent exposed portions 7 generates heat. Owing to the heat of respective heat resistance wires 5, the piping for installations are heated every zone section, so as to prevent a fluid flowing through the piping for installations from freezing.
Another conventional heating tube disclosed in Japan Patent Application Laid-Open No. Sho 56-85352 is shown as FIGS. 2(A) and 2(B).
The conventional heating tube consists of an inner layer of tube 1, a heat generating layer 2, an outer layer of tube 3, an electric heating resin film 22, two tape-like metal foil conductors 23, and a fluoroethylene plastics tape-like protective resin film 24. The metal foil conductors 23 are positioned on the electric heating resin film 22 and they are covered in turn by the protective resin film 24 in a shape of sandwich in order to form a surface heating member 21. The surface heating member 21 is wound around the tube 1 and the heat generating layer 2 is constructed. The assembly of them is covered by a tube outer layer 3.
In general, the electric heating resin film 22 above is constructed by a conductive polymer and the like having a positive thermal coefficient. Ordinarily when a voltage is impressed between the metal foil conductors 23, the higher the temperature rises, the higher the resistance becomes. An increase in the electric resistance causes the current to decrease and accordingly the temperature of the heat generating layer 2 decreases. As a result, the electric heater of this kind belongs to a type of self-control.
Such heating tube has been used, for example, as a medical tube in a medical installation. When AC is impressed to two tape-like metal foil conductor 23, the electric heating resin film 22 is heated. Heat generated by the film 22 warms the tube 1 to heat medical liquid such as dialysis liquid, and medical gas flowing through the tube 1, sustaining them at a fixed temperature.
The conventional electrical heating cable shown in FIG. 1 and described above uses a resistance wire 5 wound around the insulation shield of the feeder line 1 in spiral as an electric heating means. The resistance wire 5 mutually is connected to the core 2 at a junction point 8. When a current is fed to the resistance wire 5, a joule effect happens generating heat. When AC is impressed across the resistance wire generating heat in a certain temperature cycle and the cycle changes or an outside force such as a bending is applied to the resistance wire 5, the wire is apt to be broken thus failing to generate disadvantageously heat in a corresponding zone section. Also, when a current flows through the resistance wire 5, it is continuously heated causing its temperature to rise to a temperature zone of more than 50.degree. C. at which the wire is no longer able to sustain a load. In addition, because the single resistance wire 5 is joined to cores 2 at the junction point 8, it is possible that a bad connection happens at the point 8.
The conventional heating tube shown in FIGS. 2(A) and 2(B) are constructed as self control heaters to generate heat in the heating resin film 22 which has a positive thermal coefficient by impressing AC to two insulated tape-type metal foil conductor 23. Consequently, a stable heat generation is limited in a low temperature zone and it is disadvantageously difficult to obtain a stable generation of heat at a temperature of more than 50.degree. C.